1. Field of the Invention
The present invention relates to an apparatus which controls a motor used to drive a lens, for example.
2. Description of the Related Art
A technology of driving a lens for autofocusing of a camera by a motor, for example is conventionally known. Autofocusing requires the lens to be driven up to a target position rapidly and accurately, and there are various proposals therefor.
For example, Japanese Patent No. 2807728 specification discloses a technology of stopping a lens using a reversing brake or short brake according to the speed to stop the lens rapidly.
The short brake is a method of quickly reducing the speed of a motor by short-circuiting between the +terminal and xe2x88x92terminal of the motor and utilizing electric power generation action (counter electromotive voltage) of the motor. The reversing brake is a method of quickly reducing the speed of a motor by forcing a current to flow in the reverse revolution direction between the terminals of the motor.
Stopping the motor rapidly using such a technology can shorten the time of driving a lens, and is therefore a factor essential to autofocusing.
However, when focusing to an object from a position distant from an in-focus position is carried out, focusing is not successful by driving the lens one time due to errors of a focus detection on the camera side or optical errors, etc., and an in-focus state may be attained by driving the lens two or three times. In such a case, what is most important to shorten the time until an in-focus state is attained is to drive the lens up to the in-focus position accurately by driving the lens one time.
For this reason, there are autofocusing cameras which carry out processing called an xe2x80x9coverlap operationxe2x80x9d by carrying out focus detection and calculation once or a plurality of times after driving of the lens is started based on the focus detection result while the lens is being driven at a fixed speed. This processing allows focus detection in a place close to the in-focus position, and can thereby improve the accuracy of the focus detection result and attain an in-focus state by driving the lens one time.
Carrying out this overlap operation requires the lens to be driven at an appropriate driving speed according to the focus detection performance of the camera and brightness of the object. Therefore, it is important to accelerate the lens to a certain speed as quickly as possible and then drive the lens at a stable driving speed.
However, accelerating the motor drastically causes the motor speed to exceed a target speed, that is, an xe2x80x9covershootxe2x80x9d which requires a considerable time until the speed is stabilized to the target speed.
More specifically, in the case of conventional motor acceleration control, as shown by a curve B in FIG. 11, the speed is controlled by decreasing or increasing the voltage to be applied to the motor after an overshoot exceeding the target speed occurs so that it is finally converged to the target speed. The method of controlling the voltage applied to the motor in this way causes the driving speed to change more slowly than the applied voltage changes and takes a long time until it is stabilized to the target speed.
It is an object of the present invention to provide a motor control apparatus and lens apparatus adapted so as to accelerate the motor speed to a target speed as quickly as possible, reduce an overshoot with respect to the target speed and stabilize the motor speed in a short time.
In order to attain the above-described object, one aspect of the motor control apparatus of the present invention includes a speed detection unit that outputs a signal according to the speed of the motor or a movable unit which is driven by the motor. Furthermore, the apparatus includes a control unit which causes, when carrying out control on acceleration of the motor or the movable unit to the target speed, an electric brake to act on the motor when the speed detected based on the signal from the speed detection unit exceeds the target speed.
As the movable unit, a lens unit provided in a manner movable in the direction of the optical axis can be taken as a specific example.
Furthermore, the present invention is effective in the case where a target amount of driving, etc., of the lens unit is calculated (overlap operation) for focusing when the speed detected based on the signal from the speed detection unit is kept at the target speed at the time of acceleration in a lens apparatus or camera.
The features and advantages of the motor control apparatus and lens apparatus of the present invention will become more apparent from the following detailed descriptions of preferred embodiments of the invention with reference to the drawings.